As the demand for data storage capacity continually grows, data storage technologies are being driven to higher areal densities. A major determinant of the size and price of high-performance computers is the memory. The data storage requirements of new high-performance computers are very great, typically, many gigabytes (from 10.sup.9 to 10.sup.12 bits). New and improved, compact, low-cost, highest-capacity memory devices are needed. These memory devices should be able to store many gigabytes of information, and should be capable of randomly retrieving such information at very fast random access speeds.
The rate of growth in areal density is being led by magnetic hard disk drives, where the current annual growth rate is approximately 60%. In optical memory, such as magneto-optical data storage, which in some aspects offers an attractive alternative to magnetic storage, the growth rate in density has been slower, as the areal density is limited by the focused laser spot size.
The spot size of conventional optical storage is limited by diffraction to approximately .lambda./(2NA), where .lambda. is the free-space wavelength and NA is the numerical aperture of the objective lens. For .lambda.=0.78 micron and NA=0.45, the spot size is approximately 0.8 micron and the typical densities in practical data storage devices are of the order of Gbits/inch.sup.2.
To improve density, one possibility is to work at shorter wavelengths, typically at 488 nm, and to increase the numerical aperture to 0.64, which yields a spot size of about 0.4 micron and an area density of the order of 4 Gbits/inch.sup.2.
In order to achieve high-density reversible optical storage devices, it has been suggested to make use of the 3rd dimension in photochromic and photorefractive materials. Destructive readout (the reduction/destruction of data during readout) and crosstalk (readout noise from non-addressed data) become serious limiting factors in terms of data stability and storage density for a given data retrieval error rate (bit-error-rate BER). For example, to achieve a BER of 10.sup.-9 in volume holographic memories, it was found that the theoretical storage density was reduced by 2 to 3 orders of magnitude.
In order to reduce destructive readout and crosstalk, writing and readout by two-photon absorption have been proposed. Only where the two photons intersect, is there any writing or readout of data. This approach has several disadvantages: First, two-photon absorption only occurs at very high power densities, which can only be achieved with ultra-short (picosecond) and high-power pulsed lasers; these are very expensive and of large dimensions and therefore not suitable for commercial memory devices. Second, destructive readout and cross-talk, though reduced, cannot completely be eliminated, and are very difficult to control in the 3D space of the material. Third, the requirements on the optical properties inside the 3D material space are very demanding, as additional degradation in storage density and cross-talk will occur, originating from optical aberrations and optical scattering. Fourth, the two-photon absorption involves long wavelength writing and readout, therefore, resolution will be reduced, as predicted by the diffraction-limited spot size and depth of field.
It is thus an object of the present invention to provide a supra high-density storage device having a capacity from 50 Gbits/inch.sup.2 up to Tbits/inch.sup.2, while avoiding the above drawbacks and disadvantages of the prior art.